Enabling Microfluidics @Serkan Butun and Suhwan Choi

In this talk…

• What is microfluidics and why we use it?

• See types and applications of this technology

•What kind of methods NUFAB provides for fabricating microfluidic devices?

What is microfluidics?

• Microfluidics: Study of fluid flows and design of components that are geometrically constrained to a small scale (micrometer, µm) at which surface forces dominate volumetric forces.

• Interdisciplinary field: engineering, physics, chemistry, biology, materials science.

• Smaller length scale – different physics (ex, capillary action).

Diameter: 1d 10-1d 10-2d 10-3d

Surface Area: 1A 10-2A 10-4A 10-6A

Volume: 1V 10-3V 10-6V 10-9V

Weight: 1kG 1g 1mg 1ug

Surface/Volume: 1 10 102 103

Why microfluidics?

Low sample volume

Parallelization

Small physical

Batch fabrication

Automation

Reduce cost

High throughput experimentation

Economic footprint

Quantitative benefit

Compact

Advantages of microfluidics: Lab on a chip

Continuous-flow microfluidics

• Manipulation of liquid flow through fabricated microchannels without breaking continuity.

• Syringe pumps are typically used to pump in your reagents.

• A variety of applications including micro- and nanoparticle separators, particle focusing, chemical separation as well as simple biochemical applications.

(Quake lab, Stanford)

Droplet-based microfluidics

• Manipulates discrete volumes of fluids in immiscible phases with low Reynolds number and laminar flow regimes.

• Microdroplets allow for handling miniature volumes (μl to fl) of fluids conveniently.

• Provide better mixing, encapsulation, sorting, and sensing, and suit high throughput experiments.

• Requires a deep understanding of droplet dynamics such as droplet motion, droplet sorting, droplet merging, and droplet breakup.

(Weitz lab, Harvard)(Abate lab, UCSF)

Droplet Encapsulation

Reagent Addition

Droplet Sorting

Digital microfluidics

• An advanced fluid-handling technology that precisely manipulates droplets on a substrate using electrowetting.

• “Electrowetting” refers to the ability of an applied voltage to modulate the “wettability” of a surface.

• Harnesses electrowetting to control droplets.

• Electrical signals are applied to an array of electrodes to define the size and position of each droplet.

• Droplets are moved by turning the voltage on and off in succession across adjacent electrodes.

Paper-based microfluidics

• Paper based microfluidics rely on the phenomenon of capillary penetration in porous media.

• Hydrophobic barriers on hydrophilic paper passively transport aqueous solutions to outlets where biological reactions take place.

• Current applications include portable glucose detection and environmental testing, with hopes of reaching areas that lack advanced medical diagnostic tools.

(https://doi.org/10.1016/j.aca.2018.12.001)

(mrnok/shutterstock.com)

Photolithography @ NUFAB

Masks: Cr or Iron oxide plates Mask aligner Substrates Photoresists Developers Spinner

Required tools

Procedure

Photolithography Aligners @ NUFAB

Maskless Aligner

Contact Aligner Suss MJB4 Suss MABA6

Heidelberg uPG 501

Heidelberg MLA150

➢ Advanced contact modes➢ UV lamp –365 nm (i-line)➢ Up to 6 inch wafer process➢ Backside alignment

➢ 375, 395 and 405 nm lasers➢ Up to 150 x 150 mm writing

area➢ Backside alignment

Step 1

Step 2

Step 3

Step 4

Step 5

Microfabrication - Mold

PDMS

mask

Glass

Si

Si

SU-8

PDMS

Glass

Si

PR

mask

PDMS

Glass

❑ SU-8 ❑ Laser etching ❑ DRIE etching

SU8 Properties

MLA150 375 nm laser exposure

• Permanent Epoxy Resist• Very Stable – Chemically/Physically• Wide range of thickness• High Aspect Ratio

• Impossible to remove• Exhausting to work with• Difficult film stress control• Adhesion is poor

Pros Cons

*Also have: SU8 2002

Some SU8 molds fabricated in NUFAB

DRIE Etching @ NUFAB

• Bosch process alternating etch and passivation cycles.

• Straight side wall, highly anisotropic.

• Feature depth hundreds nanometers to 1mm.

• Highly automated machine, easy to operate, but need careful pattern layout design for desired etch profile.

High Aspect ratio devices can be easily made

DRIE bonus – PTFE deposition

• Most high aspect ratio patterns require some form of surface modification for easy detachment of PDMS from the mold.

• Typically, a chemical treatment is used.

• DRIE has a built-in PTFE deposition cycle at the end of etching that makes a highly hydrophobic non-stick surface coating.

• Easy to use, very effective.

Laser cutter @ NUFAB

• Virtually anything can be engraved or cut.

• ~20 um spot size.

• 15 um lines can be engraved or cut depending on the material thickness.

LPKF R Protolaser

Laser cut microfluidic mold fabrication approaches

1. Making Grooves on the substrate

2. Full depth cut

Rough surface

Cut the substrateall the way

Bond the piece to another substrate

Laser cut microfluidic approaches

1. Making Grooves on the substrate

2. Full depth cut

Rough surface

Cut the substrateall the way

Bond the piece to another substrate

Laser cut microfluidic approaches

1. Making Grooves on the substrate

2. Full depth cut

10 um gap at the bottom

Which method?

Channel DepthCh

ann

el W

idth

10 um 100 um 1000 um

10 um

100 um

1000 um

DRIE

LPKF R

SU8

Comparison

SU8 LPKF R DRIE

Min feature size 1 – 10 um 20 – 100 um <1 um

Aspect Ratio ~ 5 4 >10

Effort 3 1 2

Typical process time 3 hours active –including photomask making

30 min Active – actual cut time varies

1.5 hours Active

# trainings required 2 – 4MLA150Acid BenchSU8 SpinnerContact Aligner

1LPKF R

3SpinnerMLA150DRIE

Conclusion

• Microfluidics has high-throughput, quantitative, and small-scale benefits for engineering, physics, chemistry, biology, materials science, and biotechnology.

• NUFAB has a wide variety of utilizing tools for microfluidic fabrications.

• We are working hard to serve micro/nano fabrication in research.

• Please feel free to contact staff for your application questions or process development for microfluidics system.

Thank you!

Questions?